Compatibilization of polymer blends is performed to obtain synergistic effects in physical and mechanical properties. The present work demonstrates the ability of vinyltrimethoxysilane-grafted-ethylene propylene diene monomer rubber (VTMS-g-EPDM) to improve the compatibility between EPDM and polybenzoxazine (PB). EPDM reacted with 5 phr of VTMS showed the highest grafting efficiency as well as a relatively low gel content, and was added to EPDM/PB blends. The addition of 8 phr of compatibilizer to 70/30 (w/w) EPDM/PB reduced the dispersed PB droplet size from 3.1 mu m to 800 nm. Effects of various blend compositions at constant dosage of compatibilizer (8 phr) on the swelling behavior and tensile properties of the samples were also studied. The tensile strength increased from 12 to 14.5 MPa upon adding the compatibilizer at 50/50 blend ratio of EPDM/PB; however, the increase in the PB content had no significant impact on the tensile strength in both compatibilized and non-compatibilized samples.